The present invention relates to wireless systems, and in particular, to the problem of estimating the distance from a single receiver to a wireless transmitter.
Many applications make use of distance estimation for a target transmitter to some point. Many of these leverage the radio signal strength indicator (RSSI) defined in IEEE 802.11, or received channel power indicator (RCPI) signal defined in IEEE 802.11k, supplied by many devices, coupled with a reference source and a propagation model. Distance estimation is used in many fields, including security systems, asset tracking, RFID, and applications which are context and/or location aware.
The ability to identify a target transmitter's approximate distance from a single receiver is based upon several factors, often resulting in incorrect distance estimates which are skewed by unknown environmental and transmitter characteristics. Such inaccuracies reduce the effective use of this technique, as unknown factors can significantly skew distance measurement data, resulting in inconsistent and unreliable data.
One approach to estimating the distance of a target transmitter to a given receiver is based on the Free Space Path Loss (FSPL) propagation model, which gives expected path loss as a function of frequency and distance. Combining the FSPL model with a reference signal strength and a received signal strength from the target allows an estimate of the target distance to be computed. The reference signal strength, such as from RSSI or RCPI measurements, typically describes the expected signal observed by the receiver at a fixed distance of a known device (operating at known frequency and power levels). Unfortunately, this reference measurement may not represent the transmit power of the target, resulting in inaccuracies in distance estimates.
What is needed is an improved method of distance estimation for wireless devices.